The present invention relates to an RF package and, more particularly, to an RF package with a feed-through.
A package using a coplanar line is suitable for mounting an MMIC (Microwave Monolithic Integrated Circuit) by flip-chip mounting. D. R. Decker et al., xe2x80x9cMultichip MMIC Package for X and Ka Bandsxe2x80x9d, IEEE Transactions on Components, Packaging and Manufacturing Technologyxe2x80x94Part B. Vol. 20, February 1997, pp. 27-33 proposes a ceramic package which is comprised of a promisingly low-cost multilayered substrate.
FIGS. 8 to 11 show the structure of a conventional ceramic package. Referring to FIG. 8, a ceramic package 1 is comprised of a cavity 3 where a semiconductor element 2 is to be mounted, and a feed-through 4 for connecting the inside and outside of the cavity 3. As shown in FIGS. 9 and 10, the feed-through 4 is divided into a coplanar line 4a and inner layer line 4b, and is comprised of a signal conductor 5, a ground conductor 6a on the lower surface of a first layer substrate 1a, a ground conductor 6b of the coplanar line 4a, and a ground conductor 6c on the upper surface of a second layer substrate 1b (see FIG. 9).
The cavity 3 is hermetically sealed by an upper lid 7 so that it is hermetically held (see FIG. 8). In order to equalize electric potential, a plurality of via holes 8a for connecting the ground conductors 6a and ground conductor 6b to each other, and a plurality of via holes 8b for connecting the ground conductors 6a, 6b, and 6c to each other are formed along the signal propagating direction (see FIG. 9).
As shown in FIGS. 10 and 11, in the structure of the conventional ceramic package, when a distance xcex from a connection interface c-cxe2x80x2 (see FIG. 10) between the coplanar line 4a and inner layer line 4b to the center of the endmost via hole 8b formed to extend through the first and second layer substrates 1a and 1b (see FIG. 11) increases with respect to a signal wavelength, a signal is radiated in an outward propagation mode between the connection interface c-cxe2x80x2 and the endmost via hole 8b into a plane-parallel plate, constituted by the ground conductor 6b of the coplanar line 4a and the ground conductor 6c on the upper surface of the second layer substrate 1b. Studies made by the present inventors clarified that in the conventional ceramic package, the transmission characteristics in the feed-through 4 degraded due to this signal radiation.
For example, in the prior art, when a signal up to 60 GHz is to be transmitted to a feed-through made of a dielectric substrate with a specific dielectric constant of 7.1, the distance xcex must be 0.3 mm or less. To form a via hole at such a short distance degrades the yield because cracking occurs in the manufacture. For this reason, it is difficult to set the distance xcex very small (to about up to 0.4 mm), and problems arise in the yield of the manufacture, degradation in transmission characteristics of an RF signal, and the like.
It is an object of the present invention to provide an RF package with a feed-through structure in which transmission characteristics of an RF signal do not degrade.
It is another object of the present invention to provide an RF package with a feed-through structure in which the yield in the manufacture is improved.
In order to achieve the above objects, according to the present invention, there is provided an RF package comprising a multilayered dielectric substrate on which first and second dielectric substrates are formed, the multilayered dielectric substrate having a cavity where a semiconductor element is to be mounted, a feed-through for connecting an inside and outside of the cavity and comprised of a coplanar line formed on the first dielectric substrate and an inner layer line obtained by forming the second dielectric substrate on the coplanar line, the coplanar line and the inner layer line sharing a strip-like signal conductor, and metal members formed at a connection interface between the coplanar line and the inner layer line on two sides of the signal conductor.